1. Field of the Invention
The present invention relates to a self-scaling decoder for differential signals.
Whilst the invention is hereinafter described with reference to decoding signals indicative of the radially displacement of a transducer from being centrally disposed over a servo track on a disc in a disc data store, it is to be understood that this does not constitute a restriction in its use.
2. The Prior Art
It is well known to convey information by means of differential signals. Such signals consist of two components. The difference between the two components is usually the property of the signals it is desired to extract. In those cases where the difference is indicative of some measurement, it is also desirable that the difference be according to some predetermined scale. It is known that, in order that the difference between two equally scaled quantities be to a predetermined scale, the sum of the two quantities must equal some predetermined value irrespectively of their individual, instantaneous values.
To this end, it is well known to subject the two components to a common amplification process wherein an automatic level control is imposed such that the sum of the two components is equal to a predetermined level. The sum of the two amplified components is substrated from the predetermined level and the result of that subtraction used as the gain controlling signal in an automatic gain control loop.
In the case of the decoding of a servo signal recovered from a servo track on a rotating disc, a transducer picks up the servo signal which is presented as the input to a signal amplifier. The output of the signal amplifier is provided as the input to a resolver which resolves the signal into two position indicating components, the difference therebetween being indicative of the magnitude and sense of the displacement of the transducer from being centrally disposed over the servo track. The difference between the components is provided as an output, while the two components are summed in a summing amplifier. The output of the summing amplifier is provided as the subtractive input to a difference amplifier whose additive input is a predetermined fixed level. The output of the difference amplifier is coupled back to the signal amplifier as a gain controlling input. In this way the resolver provides its outputs according to a predetermined scaling factor.
The stability requirement in the feedback loop of an automatic gain control amplifier, especially one of high gain and bandwidth, such as might be used for amplifying the signals recovered from a transducer in a disc data store, is such that the bandwidth of the loop itself is kept very small. The signal must therefore be present in the amplifier for a considerable time before the output level reaches its stabilised value.
It is the trend that a servo track, in a disc data store, instead of consisting of a repeated plurality of transducer position indicating patterns recorded continuously around the disc, as was previously the case, now consists of relatively few, isolated patterns recorded among data signals and designed to provide a decoded output indicative of the radial position of that data, for use as a position feedback to a transducer positioner. One isolated pattern, such as might be found among the data, is insufficient to stabilise an automatic gain control loop, and therefore cannot provide an accurately scaled output when used as the input to the type of decoder already known in the art. Uncertainty of scale makes for difficulties in using the decoded servo signal as a feedback signal for the transducer positioner, since the gain and phase margins of the positioning loop so obtained are rendered uncertain. If an automatic gain control loop is so used, an additional problem arises in that the low bandwidth of the loop introduces additional, unwanted poles and zeros into the positioning loop response. These problems are accentuated by the differences in recovered amplitude encountered by a transducer when recovering signals from different radii on the disc.
It is therefore desirable to provide a decoder capable of providing an accurately scaled output from an isolated servo pattern recorded on a disc without the introduction of low frequency poles.